5.1 Parallel Installations

Guile provides strong API and ABI stability guarantees during stable
series, so that if a user writes a program against Guile version 2.0.3,
it will be compatible with some future version 2.0.7. We say in this
case that 2.0 is the effective version, composed of the major and
minor versions, in this case 2 and 0.

Users may install multiple effective versions of Guile, with each
version’s headers, libraries, and Scheme files under their own
directories. This provides the necessary stability guarantee for users,
while also allowing Guile developers to evolve the language and its
implementation.

However, parallel installability does have a down-side, in that users
need to know which version of Guile to ask for, when they build against
Guile. Guile solves this problem by installing a file to be read by the
pkg-config utility, a tool to query installed packages by name.
Guile encodes the version into its pkg-config name, so that users can
ask for guile-2.0 or guile-2.2, as appropriate.

For effective version 2.0, for example, you would
invoke pkg-config --cflags --libs guile-2.0
to get the compilation and linking flags necessary to link to version
2.0 of Guile. You would typically run
pkg-config during the configuration phase of your program and use
the obtained information in the Makefile.

The default directory where Guile looks for Scheme source and compiled
files (see %site-dir). Run
pkg-config guile-2.0 --variable=sitedir
to see its value. See GUILE_SITE_DIR, for more on
how to use it from Autoconf.

extensiondir

The default directory where Guile looks for extensions—i.e., shared
libraries providing additional features (see Modules and Extensions). Run pkg-config guile-2.0
--variable=extensiondir to see its value.